1. Field of the Invention
The present invention relates to a liquid droplet detecting device, an inkjet recording device incorporating such a device and a liquid droplet detection method.
2. Description of the Related Art
An inkjet recording device to form an image on a paper by discharging ink from nozzles of recording heads is known. Nozzle discharge failure may occur in this device due to dried ink or entry of foreign matter and air bubbles.
Such nozzle discharge failure results in deteriorating image quality. In view of this, there is a known inkjet recording device with a liquid droplet detector which determines presence or absence of ink droplets from nozzles of a recording head by emitting a light beam to ink droplets from the nozzles and receiving scattered light by the droplets in a light traveling direction.
Another known inkjet recording device is a line inkjet printer in which recording heads having nozzles are arranged on a straight line along a paper width to discharge liquid droplets from the nozzles, aiming for improving image quality and increasing printing speed.
A known line inkjet printer comprises light emitting elements arranged in a paper conveying direction to emit light beams, to detect liquid droplets from nozzle lines arranged both in a paper width direction and the paper conveying direction orthogonal to the paper width direction.
In this printer the light emitting elements to emit light beams and light receiving elements to receive scattered light of the light beams are arranged separately with a distance corresponding to a paper width. Because of this, this printer is likely to be affected by reflected light and diffracted light so that it is difficult to accurately detect scattered light.
Moreover, an error in the optical axes of the light emitting elements and light receiving elements is large since both of the elements are separated. In view of this, a technique has been developed for detecting ink droplets at a proper position by decreasing the size of light beams, enhancing drive outputs of the light emitting elements and moving the light emitting elements in a paper conveying direction.
It is possible to detect liquid droplets at a proper position by moving the light emitting elements. However, there may be an error in the nozzle arrangement in the recording heads and an error in the directions in which ink droplets are discharged. To check presence or absence of a liquid droplet for all the nozzles by turns, it is necessary to reciprocate the light emitting elements in accordance with the positions of the nozzles to emit the light beams in a reciprocative manner in the paper conveying direction.
Along with a repetition of reciprocated scanning, an error in the stop positions of the light emitting elements will occur, causing target liquid droplets to be outside in the detection width of the light beam. Accordingly, an offset between a detected liquid droplet and a nozzle discharging this liquid droplet will occur. This problem can be dealt with by emitting light beams obliquely relative to the arrangement of the nozzles to reduce the errors in the stop positions of the light emitting elements and in the droplet discharge direction and moving the light emitting elements in one direction.
However, there is a drawback in the oblique light emission that the number of liquid droplets in the detection width of the light beam is decreased. This increases the moving area of the light emitting elements and requires a large amount of time for the liquid droplet detection.
In particular, an inkjet recording device used for printing high-speed continuous stationary comprises fixed recording heads for the purpose of printing total print area along a paper width at a certain resolution at high speed. The paper width is as large as 21 inches.
For example, for printing on a paper 21 inches wide at a recording density of 600 DPI, 12,600 nozzles per line are needed in the paper width direction.
Thus, this type inkjet recording device requires an enormous amount of time to detect liquid droplets from all of the nozzles by a light emission with the number of droplets in the detection width of the light beam decreased.
Further, a light emitting element unit and a light receiving element unit have to be individually mounted in the recording head unit. Therefore, it is difficult to position the light emitting elements and light receiving elements oppositely with a mounting error taken into account.
Furthermore, Japanese Laid-open Patent Application Publication No. 2006-110964 discloses a technique to concurrently detect ink droplets discharged from nozzles from a change in outputs of a received light beam by obliquely emitting a light beam relative to nozzle arrangement so as not to overlap ink droplets in a cross section of received light and controlling timing at which the ink droplets are discharged. This device can shorten time for determining presence or absence of discharged ink droplets.